(1) Field of the Invention
This invention relates to an apparatus for selecting the field of view of a specimen or sample, and more particularly to an apparatus for selecting the field of view of a sample as is suitable for use in a scanning electron microscope, an electron probe X-ray microanalyzer, an ion microanalyzer, etc.
(2) Description of the Prior Art
In the scanning electron microscope, the electron probe X-ray microanalyzer etc., a sample is scanned in two dimensions by an electron beam, and the image of the scanned region of the sample is displayed on an displaying device on the basis of an information signal peculiar to the sample (for example, secondary electrons, backscattered or reflected electrons, absorbed electrons, Auger electrons, X-rays, cathode luminescence, etc.) as obtained from the sample by the scanning. Likewise, in the ion microanalyzer, the display of the image of a sample is often performed. In this case, however, a beam to irradiate or bombard the sample is an ion beam, and secondary ions are often employed as the information signal peculiar to the sample as obtained therefrom.
In such apparatus, it is necessary to frequently select that region or position of the sample for observation which is to be irradiated by the irradiation beam, in other words, to frequently select the field of view of the sample. The method for the selection includes the so-called mechanical system in which the sample is moved, and the so-called electrical system in which the sample is kept stationary and the irradiation beam is electrically deflected. Both the systems have merits and demerits. By way of example, when the electrical system is employed where the range of selecting the field of view is very wide or where the magnification for the observation is very low, the irradiation beam need be largely deflected, and hence, it is sometimes impossible to neglect the problem of aberration. From such standpoint, accordingly, it can be said that the mechanical system is more advantageous. The mechanical system, however, is attended with such problems that the use of a lubricant which is necessary for smoothly driving the sample but which is unfavorable for a vacuum equipment is inevitable, that the reliability lowers due to the abrasion of moving elements, and that a complicated sample driving mechanism which is a hindrance to the high vacuum of the surroundings of the sample must be arranged around the sample. Accordingly, in a range in which the problem of aberration is negligible, it is advantageous to employ the electrical system which is less subject to the problems than the mechanical system.
A conventional, sample-view selecting apparatus of the electrical system comprises a delfection coil, a current source which converts a voltage into a current and which supplies the current to the deflecting coil, and potentiometers for changing the voltages. As the potentiometer, there is generally used one which consists of a wound type slide resistance and slider adapted to slide on the surface thereof. When the potentiometer is regulated to change the current flowing through the deflecting coil, the irradiation beam to irradiate the sample is deflected accordingly. In this way, therefore, the field of view of the sample can be selected.
In such electrical, sample-view selecting apparatus, however, the lowering of reliability and the shortening of life ascribable to the abrasion of the slide portions of the potentiometer are serious problems because the operation of selecting the field of view of the sample must be frequently carried out.